The present invention relates to devices for injecting, administering, delivering, dispensing or infusing substances, and to methods for making and using such devices. More particularly, the present invention relates to an injection device with a spring, which serves as a driving spring for a conveying mechanism of the injection device. The present invention further relates to a spring used as a compression spring, e.g., a driving spring, for an injection device. The injection device may be an injection pen for administering insulin, growth hormones or an osteoporosis preparation in particular.
Mechanical springs, e.g., coil springs used as compression springs, are often used in injection pens as a means of driving a plunger in a container filled with the product to be injected in a forward drive direction, generally the distal direction. Such use as a driving spring is known from applications based on automatic injection devices in particular, which represents one type of injection device for the purpose of the present invention. Driving springs of well known automatic injectors are used not only to drive the plunger forward, but also to drive the injection needle during the injection. This being the case, the spring is therefore not only relaxed or the tension therein released by the distance over which the plunger has to be driven, but also by the length over which the needle has to be driven during the injection. In such cases, the driving spring of an automatic injector is longer than a driving spring which is used to drive the plunger only wherein the respective product containers must naturally be of the same length. The longer and slimmer the injection spring is, the more critical bending or buckling stresses can be, particularly in the case of automatic injectors, because the spring is typically required to expend a higher spring force in the forward drive direction than springs which only have to drive the plunger due to the fact that it has to drive the needle.
An automatic injector of the type to which the present invention relates in particular, although not exclusively, is known from German patent application No. 103 51 594. With this automatic injector, the plunger rod is provided in the form of a sleeve with a base or shoulder at its distal end and the driving spring projects into the sleeve in the distal direction onto the base or shoulder. Depending on the type of product container, the plunger rod sleeve is correspondingly slim and the spring is therefore slimmer. To prevent buckling or even just an outward deflection of the spring, the plunger rod co-operates with a support structure on which the spring is supported in the proximal direction to form a telescopic guide for the spring. The plunger rod and a guide portion of the support structure extend by an appropriate length in the distal direction in order to continue providing a reliable guide for the plunger rod even when the plunger rod has assumed its most distal position, having emptied the container.